The present invention relates to a method for coating a flexible support (hereinafter referred to as a "web") made of plastic film, paper, metal foil, or the like with a coating solution such as a photographic photosensitive solution, magnetic solution, surface protective solution or the like, and an apparatus for practicing the method.
Examples of a conventional coating apparatus of this type are a roll-type coating apparatus, a bead-type coating apparatus, a slide-type coating apparatus, and an extrusion-type coating apparatus. In all of these coating apparatuses, the width of the coating head is larger than the width of the web being coated, and the width of the coated area on the web is less than the width of the web. Hence, when the web is coated with such an apparatus, the web will have portions along both its edges on which no coating solution is applied. (These areas are hereinafter referred to as "coating-solution-free portions" when applicable.) The coating-solution-free portions are each typically about five or six millimeters to about fifteen or sixteen millimeters in width.
In the case of an extrusion-type coating apparatus, such as disclosed by Japanese Unexamined Published Patent Applications Nos. 84771/1982 and 94657/1984, a running web is coated with a coating solution with a coating head pushed against it. Since the gap between the web and the coating head is extremely small, the above-described coating-solution-free portions are brought into contact with the coating head.
As a result, the coating-solution-free portions of the web are scraped by the coating head, thus causing the generation of fine scrapings. These scrapings have a tendency to stick to both ends of the coating edges of the coating head. In addition, foreign matter on the surface of the web may be caught by the coating edge of the coating head.
These waste materials accumulated at both ends of the coating edge raise both edges of the web. As a result, the thickness of the coating solution applied to the web is not uniform in the widthwise direction. In other words, the resultant layer on the web is thicker in both edge regions than in the middle region. This difficulty is especially noticeable when the coating layer is thin (a coating quantity of 30 cc/m.sup.2 or less) or when the coating rate is high (200 mm/min or higher).
In order to overcome the above-described difficulty, in a coating method disclosed in Japanese Unexamined Published Patent Application No. 257268/1986, before a coating solution is applied to a flexible support, a solvent is applied to both edge regions of the support located at both sides of the region which is to be coated with a coating solution (i.e., the aforementioned coating-solution-free portions). That is, a pre-coating layer is formed in the coating-solution-free regions. For the same purpose, Japanese Unexamined Published Patent Application No. 257263/1986 has disclosed a coating apparatus in which the top of a doctor edge confronted with the coating width regulating board is chamfered.
However, the coating method disclosed by the Japanese Unexamined Patent Application No. 257268/1986 is still disadvantageous in that, because of external disturbances such as the fluttering or meandering of the web which may be caused when it is run, the solvent is liable to run into the coating region of the web which is or to flow over to the other side of the web. That is, the coating solution may be applied nonuniformly to the web. Also, the pre-coating layers are increased in width by the back edge and the doctor edge (provided on the coating head end). Therefore, if the coating-solution-free portions are relatively small in width, or the scraping force of the doctor edge is large, then it is rather difficult to suitably control the applied width of the solvent.
If the above-described difficulty of the solvent flowing out of the coating-solution-free regions of the web occurs in the manufacture of a magnetic recording medium, the magnetic coating solution will adhere to the conveying rolls driving the nonmagnetic support of the web, thus smudging the nonmagnetic support and the calender roll in the following surface treatment section. As a result, the surface of the support is often scratched in the surface treatment section by foreign matter. Thus, the resultant product often has unsatisfactory magnetic characteristics.
The coating apparatus disclosed by Japanese Unexamined Published Patent Application No. 257263/1986 is also disadvantageous in that it is impossible to prevent the coating-solution-free portions of the web from being brought into contact with the back edge, and the back edge scrapes the support or catches foreign matter. Especially if the web is thin (40 .mu.m or less), it is low in rigidity, and accordingly both edge portions are strongly bent toward the ends of the coating head. As a result, the edge portions of the web are brought into contact with the edges of the slope or step formed at the ends of the coating head, whereby the web may be scraped.
In order to eliminate the drawbacks accompanying the above-described coating methods and coating apparatuses, Japanese Patent Application No. 201996/1988 has disclosed a coating apparatus in which slopes or steps are formed in the two end portions of the coating head which confront the coating-solution-free portions of the flexible support. The slopes or steps are spaced away from the flexible support, i.e. they extend from the middle of each of the end portions of the coating head toward both sides, so that there are gaps between the end portions of the coating head and the coating-solution-free portions of the flexible support. The apparatus can eliminate the difficulties of the coating-solution-free portions of the flexible support being scraped by the coating head and foreign matter on the flexible support being caught by both end portions of the coating head.
However, in the case where the web is thin (15 to 40 .mu.m in thickness) and hence low in rigidity, both edge portions of the web have a strong tendency to bend towards the ends of the coating head. As a result, the end portions of the web are brought into contact with the slopes or steps provided at the ends of the coating head, whereby the web may be scraped.
Recently, sometimes webs for VTR (video tape recorder) magnetic recording media employ a PET (polyethylene terephthalate) base to which is added a filler of spherical SiO.sub.2 material in order to reduce the contact resistance of the manufactured magnetic recording media with contacting components in the VTR, such as guide poles and the like, thereby to permit more stable running of the magnetic recording media. On the other hand, a PET base with added filler is lower in rigidity. Therefore, in applying a coating solution to such a base with the above-described coating apparatus, both edge portions of the PET base are liable to strongly bend towards the ends of the coating head. When the PET base is brought into contact with the end portions of the coating head, the filler is liable to be removed therefrom, and the resulting scrapings from the web tend to accumulate at the end portions of the coating head.